1. Field of the Invention
This invention relates to a Karman vortex flow meter which detects a generation frequency of Karman vortex streets with an ultrasonic wave to measure a flow velocity or flow rate of fluid flowing in a conduit.
2. Description of the Prior Art
Conventionally, a Karman vortex flow meter of the type mentioned is disclosed, for example, in Japanese Utility Model Publication No. 48-17010, Japanese Patent Publication No. 55-11206 or Japanese Patent Publication No. 56-34046. Those devices are each constituted such that an ultrasonic wave generated by an ultrasonic wave transmitter is received by a receiver.
It is proposed in Japanese Patent Publication No. 56-34046 to dispose a transmitter and a receiver at displaced locations side taking into consideration that an ultrasonic wave which is emitted from the transmitter and propagated across fluid flowing in the conduit is deflected to the downstream side due to a flow velocity of the fluid.
Since the conventional Karman vortex flow meters have such a construction as described above, if the receiver is disposed, for example, at a position at which it can receive an ultrasonic wave propagating thereto through a flow with its highest sensitivity when the flow velocity of fluid flowing in the conduit is comparatively low, then when the flow of the fluid becomes high, an ultrasonic wave which is deflected by the flow cannot be received with a high sensitivity.
Therefore, it is a common practice that the output power of an ultrasonic wave of the transmitter is enhanced or that the location of the receiver is modified so as to facilitate reception of the ultrasonic wave.
However, when the first method is taken, in other words, if a voltage higher than a rated value is applied to the transmitter to increase the output power of an ultrasonic wave, then the transmitter will sometimes suffer from deterioration in characteristic or from failure. Further, since a power supply voltage (terminal voltage of a battery) is normally comparatively low in such a case as the Karman vortex flow meter applied to detect an intake air amount of an engine for an automobile, the voltage to be applied to the transmitter cannot readily be raised to a level higher than the power supply voltage. If the second method is taken, in other words, if the receiver is disposed at a position where the reception sensitivity thereof is in the highest condition when the flow velocity is high, then when the flow velocity becomes low, the reception sensitivity is deteriorated.